In many areas of technology the problem arises of determining as accurately as possible the angular position of an object rotating about a rotation axis. One widely used method in this respect consists in providing the rotating object with a suitable coding pattern that is sensed by means of a sensor in order thus to obtain items of information about the angular position of the rotating object. In this case, the detection of the coding pattern by the sensor is often effected contactlessly, for example with the use of a magnetic coding pattern and a magnetosensitive sensor.
One known method for identifying the angular position of an object rotating about an axis A is explained below with reference to FIGS. 1 and 2. In this case, a number of bar magnets 3, 4 are arranged around the periphery of the object 1 rotating about the rotation axis A, the longitudinal axis of said bar magnets being arranged in the radial direction of the rotating object 1. The rotating object 1 has a coding pattern dependent on the orientation of the individual bar magnets 3, 4. The individual bar magnets 3, 4 generate magnetic fields that are superposed to form a total magnetic field. The total magnetic field is sensed by a sensor 2 during a rotation of the rotating object about its axis in order to deduce from this the angular position of the rotating object with respect to the sensor 2.
The sensor 2 is a magnetic field sensor 9, for example a Hall sensor 8.
Referring to FIG. 1b, teeth 5, 6 similar to the teeth of a gearwheel may also be arranged around the periphery. Said teeth 5, 6 generally have different width, however. Accordingly, the gaps between two teeth 5, 6 spaced apart from one another in the peripheral direction are generally also of different sizes. The teeth 5, 6 and also the intervening gaps form a coding pattern of the rotating object 1.
The coding pattern is sensed by means of a sensor 2 comprising a magnetic field sensor and also a magnet 7.
The rotating object 1 is formed from magnetic material, so that the magnetic field that issues from the magnet 7 and is altered during the rotation of the rotating object 1 in a manner dependent on the coding pattern formed by the teeth 5, 6 can be ascertained by means of the magnetic field sensor 9.
The magnetic field sensor 9 shown in FIGS. 1 and 1b is a Hall element or a coil, for example. The sensor 2 has an analog output, which may be designed either as a differential output or as a single-ended output.
If the rotating object 1 shown in FIG. 1b rotates about its axis A in the direction of the arrow, then firstly the tooth 5 and then the tooth 6 passes the sensor 2. The tooth 5 is considered by way of example in the following text. The tooth 5 has a first tooth flank 51 and a second tooth flank 52. Upon rotation of the rotating object 1, firstly the first tooth flank 51 and then the second tooth flank 52 passes the sensor 2. The sensor outputs a signal each time a tooth flank 51 and 52 passes it.
FIG. 2 illustrates the typical profile of a signal S1 of this type as is output by the sensor 2 if the tooth flanks 51, 52 pass said sensor 2 in temporal succession. The sensor signal S1 represents a summation signal formed by the superposition of a plurality of partial signals, of which the partial signals S1a and S1b are illustrated by way of example.
The partial signal S1a corresponds to the signal component brought about by the flanks 51, that is to say corresponds to a signal that would be output by the sensor 2 if the rotating object 1 had merely the tooth flank 51 as a single tooth flank instead of a multiplicity of tooth flanks 51, 52. The profile of this partial signal S21 corresponds to a Lorentz curve.
The partial curve S1b correspondingly indicates how the signal S1 output by the sensor 2 would appear if the rotating object 1 had merely the second partial flank 52. The other teeth 6 of the rotating object 1 also correspondingly have tooth flanks 61, 62, each of which can be assigned a partial signal in the manner described. The sensor signal S1 arises from the superposition of all these partial signals.
The profile of the sensor signal S1 in the region of tooth flank 51 is thus determined not only by the partial signal S1a of the tooth flank 51 but also by the partial signal S2a of the tooth flank 52 and by the partial signals of the tooth flanks of adjacent teeth.
As can be seen from FIG. 2, the sensor signal S1 deviates from the profile of the respective partial signals S21, S22 on account of said superposition primarily in the region of the tooth flanks 51, 52. A specific partial signal is influenced in particular by the relatively close vicinity of the tooth flank assigned to the relevant partial signal. Since the teeth arranged around the periphery of the rotating object may have not only identical but also different tooth width and tooth spacings, the partial signals of different tooth flanks are generally influenced to different extents by the respective vicinities of the relevant tooth flanks, so that, proceeding from the sensor signal S1, the exact angular position of the rotating object 1 with respect to the sensor 2 can initially be ascertained only within the scope of the deviations described.
In order to improve the accuracy when determining the angular position of the rotating object, a method such as is illustrated on the basis of a block diagram in FIG. 3 has been utilized hitherto. In this case, the signal S1 supplied by a sensor 2 is firstly subjected to a signal conditioning by a signal conditioning unit 98, which essentially comprises a signal waveform distorter 90 and an amplitude distorter 92. The signal conditioning unit 98 serves for reducing the noise above the band limit.
The sensor signal S1 conditioned by the signal conditioning unit 98 is subsequently equalized by means of a feed forward equalizer (FFE) 93 and a downstream decision feedback equalizer (DFE) 94. The FFE 93 and the DFE 94 together form a zero forcing decision post-equalizer (zDFE) 99.
The output signal of the FFE 93 is fed to a subtractor 94a of the DFE.
The output of the substractor 94a is fed to a threshold value decoder 94b, which generates an output signal SA of the circuit.
Furthermore, said output signal SA is fed to a feedback filter 94c, which filters the output signal SA and feeds the filtered output signal SF to the subtractor 94a. As a result, the difference signal between the output signal SA and the filtered output signal SF of the circuit is fed to the threshold value decoder 94b thereby forming a signal feedback within the DFE 94.
The signal processing within the DFE 94 is effected in clocked fashion in such a way that, per clock period, a filtered output value S(k−1) of the filtered output signal, said value being ascertained during the previous clock period, is subtracted from an output value S(k) of the output signal of the FFE 93.
Since the output signal S(k−1) of the preceding clock period k−1 is generated from the coding pattern of the rotating object 1, the output signal S(k−1) of the preceding clock period k−1 contains an item of information about the coding pattern that influences the sensor signal S1 during the current clock period k on account of intersymbol interference (ISI).
By virtue of the filtering and the feedback of the output signal S(k−1) output during the preceding clock period k−1, it is thus possible, given a suitable configuration of the feedback filter 94c, to effect an improved determination of the angular position of the rotating object 1 with respect to the sensor 2, since the intersymbol interference is thereby eliminated to a certain degree.
Such methods and arrangements for suppressing intersymbol interference are described for example in Le, M. Q. et al.: “An Analog DFE for disk drives using a mixed-signal integrator”, IEEE Journal, May 1999, VOL. 34 NO. 5, pages 592–598, or on the Internet under http://www.ece.ucdavis.edu/˜hurst/papers/Le, JSSC99.pdf.
For these and other reasons there is a need for the present invention.